Starters for internal combustion engines are essentially comprised of an electric motor that acts by means of a reduction gear on the crankshaft of the internal combustion engine and, within an interval of for example 0.5 to 1 sec, accelerates it to speeds of typically more than 200 rpm. The output required for this ranges from less than 1 kW for small gasoline engines to greater than 4 kW for large diesel engines. Because of the extremely high currents ranging from several hundred to over 1000 A temporarily required for this, and due to the limited, fixed internal resistance of the battery and the altogether low all-in resistance of the starter circuit, the battery voltage drops from for example 12 V down to 3 V, as a result of which the power supply to most of electronic devices in the vehicle can be interrupted.
Whereas this interaction of the starter with other vehicle components is hardly significant when it occurs once at the beginning of a trip, the consequences of the drop the electrical system voltage with repeated starting in so-called start and stop operation are intolerable and under certain circumstances, can even be safety critical. There are various conceivable strategies for ameliorating these consequences in order to avoid or suppress this drop in the electrical system voltage. For example, these measures include an electrical system with two batteries of the kind known, for example, from DE-OS 41 38 943. Another measure is the regulation of the starting current by means of a so-called MOSFET switch. The complexity of these measures, however, necessarily adds considerable expense to the electrical system or the starter.
In small motor construction and the use of small motors in vehicles, a known method for limiting the starting currents of these small motors is to connect resistors with negative temperature coefficients, so-called NTC resistors, before the electromagnetically active part of the small motor. After the small motor is switched on, the NTC resistor is powerfully heated by the current load and the initially high resistance. Due to the negative temperature coefficient of the resistor, this initially high resistance decreases and simultaneously so does the power loss of the resistor so that even the losses in stationary operation are low. German patent application DE 41 22 252 A1 has disclosed a circuit of this kind with an NTC resistor. In it, a so-called NTC resistor is built into a secondary current path of the starter device. This NTC resistor—which is known in this location—is connected in parallel with the main current path of the electric motor, is simultaneously connected in series with the parallel-connected pull-in and holding coils of the starter relay, and influences the current load during starting.
The commercially available NTC resistors, i.e. resistors with negative temperature coefficients, are based on a ceramic substrate and are comprised of semiconducting ceramic with a comparatively low current-carrying capacity. It would be impractical to use these components in a range of approximately 1000 A since they would then have to be extremely large and would therefore be unstable in certain circumstances. Such NTC resistors would also be geometrically too large for conventional starter devices.
The maximum voltage drop in the electrical system during operation of the starter is determined by the internal resistance of the battery, the ohmic resistance of the armature winding of the starter motor, and the voltage drop at the carbon brushes. This operating condition occurs at the instant in which the starter is in fact being supplied with current, but the rotor of the starter has not yet begun to rotate. This instant is also referred to as the short-circuit point; the short-circuit current is then flowing. With increasing speed, the voltage induced in the windings of the starter reduces the flux and therefore the voltage drop in the electrical system.
The object of the present invention is to limit the current, in particular the short-circuit current, while simultaneously achieving a low or minimal power loss during cranking operation. The current limitation here should be achieved at a minimal expense and should be possible to produce in the form of a component of the starter. This object is attained by means of an electric motor used as a starter, with the combination of characteristics cited in the main claim.